Job Knowledge 85
This article looks at the wire consumables used in the gas shielded MIG/MAG, metal cored (MC) and flux cored (FC) arc welding processes.
The MIG/MAG processes were first developed using a solid wire but some 25 years ago tubular wires began to be supplied and since then the use of these wires has grown rapidly and they now form a significant proportion of the welding wire market - cored wires are now used not only in the MIG/MAG process but also in TIG, plasma-TIG and submerged arc welding.
Solid wire for welding of alloy steels is an expensive commodity. The composition of a ferritic steel welding wire is not the same as that of the steel that it will be used to weld. The ingot from which the wire is drawn must contain all the de-oxidation and alloying elements that can be contained in the flux on an MMA electrode.
Steel is produced most economically in large tonnages whereas a consumable supplier requires only relatively small amounts and these requirements have a significant effect on the cost. In addition, it can be difficult to draw down the wire to the small diameters required for welding.
Cored wires for welding carbon and alloy steels, however, can be made from mild steel with the alloying elements added to the flux filling. This enables small amounts of wire to be economically produced matching the composition of steels where the usage is limited, eg high chromium creep resistant steels or hard facing. Non-ferrous and austenitic steel wires, aluminium, nickel based, stainless steel etc however, generally match closely the parent metal composition and obtaining ingots for drawing into wire is less of a problem.
MIG/MAG welding solid wires are provided in diameters ranging from 0.6 to 2.4mm, the most commonly used diameters being 1.2 and 1.6mm.
As mentioned above, the solid wires are generally formulated to match the composition of the alloy to be welded. Silicon, 0.5 to 0.9%, and perhaps aluminium, up to 0.15%, are added to ferritic steel wires to provide de-oxidation; carbon content is generally below 0.1%.
Alloying elements such as manganese, chromium, nickel and molybdenum are added to the ingot to provide improved mechanical properties and corrosion resistance. In addition the carbon and low alloy steel wires are often copper coated, both to reduce corrosion during storage and to improve welding current pick-up in the contact tip.
The stainless steel and non-ferrous wires are not copper coated. Poor control during the drawing operation may form laps on the wire surface that trap contaminants and give rise to porosity, as can a poor quality copper coat on ferritic steel wires.
Porosity from drawing defects can be a particular problem with aluminium alloy wires and where high quality weld metal is required, then shaving the wire to remove defects on the wire surface is recommended.
The cored wires are small diameter tubes in which are packed fluxes and alloying elements. There are two fundamental types, one containing mostly fluxes, the other containing metal powders. There is a sub-class of the flux cored wires, the self-shielded wires, that contain gas-generating compounds that decompose in the arc to provide enough shielding gas so that additional gas shielding is not required.
In cross-section, the wires may be seamless tubes packed with the flux and extruded before being drawn into a wire. Alternatively, they may be or made by rolling a flat strip into a 'U', filling this with the flux or metal power and then folding this into a tube. The edges of the tube may be butted together or overlapped.
The seamless and closed butt wires tend to have thicker walls and therefore less fill than the overlapped wires, perhaps as little as 20% of cross sectional area compared with 50% for the overlapped wires. This enables the overlapped wires to contain more alloying elements and they are therefore often used for stainless steel and hard facing welding.
Cored wires have a number of advantages over the solid wires. The reduced current carrying cross-sectional area of the wire results in greater current density and an increase in burn-off rate with increased deposition.
The flux also produces a slag that will control weld bead shape enabling higher welding currents to be used in positional welding than can be used with MAG. A 7mm throat fillet is possible in the horizontal-vertical position, for example. The slag will also react with the weldpool and provide better properties than can be achieved with MAG. Good Charpy impact properties down to -50°C are achievable in carbon steels with the correct wire.
Disadvantages with the cored wires are:
- The wire is mechanically weak and over-pressure on the wire drive rolls may crush the wire preventing it from feeding through the contact tip.
- The flux cored wires produce a slag that must be removed.
While solid wires often produce islands of a glassy slag that tend to lie in the finish craters this does not necessarily prevent a multi-pass weld being made without de-slagging.
This is not possible with flux cored wires, restricting their use in applications such as robotic welding to single pass welds. Metal cored wires are less of a problem in this context and are often used in fully automated, multi-pass applications.
As with MMA electrodes, the flux in the core may be either rutile or basic, the rutile flux providing a smooth arc, easy slag removal and 'welder appeal', the basic fluxes providing better mechanical properties and cleaner radiographic quality welds.
Hydrogen control is less of a problem than with MMA electrodes. Both rutile, basic and metal cored wires all have very low hydrogen potential levels, allowing lower preheat than might otherwise be the case and enabling rutile wires to be used in applications such as the welding of high strength or thick section steels. Hydrogen pick-up on the shop floor is also less of a problem as the flux/metal powder is contained within a sealed tube, preventing moisture ingress. Seamless wires tend to be better in this respect than seamed wires.
There are a number of specifications detailing the requirements for solid and cored wires for MIG/MAG, FCA and MCA welding and these will be covered in the next article.
This article was written by Gene Mathers.